(1) Field of the Invention
The present invention relates to a transmission and reception system for data broadcast that includes a transmission apparatus for transmitting data broadcasting contents, and a receiving apparatus for receiving the transmitted data broadcasting contents and presenting them to an audience.
(2) Description of the Related Art
In the satellite digital broadcast, which has been put to practical use in recent years, audiovisual data is distributed in the transmission format in conformity with a Moving Picture Experts Group 2 (MPEG2) transport stream specified by the ISO/IEC13818.
Various types of data including audiovisual data can be multiplexed into the MPEG2 transport stream. A data broadcasting system using this MPEG2 is nearing practical use. In this data broadcasting system, MPEG2 transport streams, in which data broadcasting contents relating to audiovisual data, and including characters, graphics, buttons, etc. are superimposed on mainly pictures, are multiplexed and broadcasted.
In a data broadcasting transmission/reception system for transmitting and receiving the data broadcast, a transmission apparatus repeatedly transmits a cluster of data broadcasting contents, while a receiving apparatus receives a data broadcasting content in response to selection by a viewer and presents it to the viewer. Thus, virtual interaction can be realized.
Contents of data broadcast, for example, include a news program as audiovisual data and commentary information relating to the news and the like as data broadcasting contents, or a commercial as audiovisual data and information and the like for enabling an audience to purchase the product in the commercial as data broadcasting contents.
Once a receiving apparatus for data broadcast receives data broadcasting contents and stores them in the internal memory, it performs a series of processes: to access each type of data such as characters and graphics of the data broadcasting contents, perform arithmetic computations on calculation of the two-dimensional coordinates on the display screen for the characters and graphics, expand the data such as characters and graphics in a frame memory for storing images, and output image signals to a display or the like in response to the data in the frame memory by the time when they should be presented.
Normally, it takes more than a few seconds for the receiving apparatus to receive data broadcasting contents and store them in the memory. In addition, after storing the data broadcasting contents in the memory, it takes approximately a few seconds to complete the series of processes required until the two-dimensional coordinates on the screen for character, graphics, and so on are calculated. The time required for the processes varies with the contents of characters, graphics, and so on.
Therefore, it becomes difficult for the transmission apparatus to precisely control the timing when the receiving apparatus presents the data broadcasting contents.
For instance, suppose that audiovisual data to be broadcasted is switched from a news program to a commercial and the transmission apparatus starts to transmit the data broadcasting contents relating to the commercial concurrently with the timing for transmitting audiovisual data for the commercial following the news program. This cause a time lag between the timing when the receiving apparatus starts to present the data broadcasting contents relating to the commercial and the timing when the receiving apparatus starts to present the audiovisual data for the commercial. To cope with this problem, it might be preferred that the transmission apparatus starts to transmit the data broadcasting contents relating to the commercial a bit earlier than the start time for the commercial. However, this is not a feasible solution because it is extremely difficult to determine how earlier the data broadcasting contents should be transmitted than the audiovisual data so that all of them can be presented at the same time.